Musar do Liczb 12:2

And we learn in Sifrei (Devarim 24:8): "Be heedful of the plague-spot of leprosy, etc.," followed by (Ibid. 9): "Remember what the L-rd your G-d did to Miriam, etc." What is the rationale of the juxtaposition? We are hereby taught that she was punished only because of lashon hara [(Miriam having slandered Moses because of the Kushite woman that he had taken.)] Now does this not follow a fortiori? viz.: If Miriam, who had spoken thus only of her younger brother, was punished, then one who speaks thus of someone greater than he, how much more so [should he be punished!] Another interpretation: Now, if Miriam, who, when she spoke, was heard by no one but the Holy One Blessed be He alone, as it is written (Numbers 12:2): 'And the L-rd heard,' was punished thus, how much more so, one who speaks demeaningly of his friend in public!"